Swine flu threat: scientists race to find out how and why

This is arguably the most fraught time for those charged with tracking and responding to the swine flu outbreak. The virus is spreading rapidly, but it is so poorly understood that the true extent of the threat is desperately hard to judge.

Swine flu is nothing new. The first virus in the US was isolated in 1930, and since then there has been roughly one human case in the country every year. Only two people have died from the infection since 1976.

The latest virus is certainly more infectious than previous strains, but little else is known for sure. Officials at the World Health Organisation and elsewhere are still working out how often it kills, how swiftly, and even precisely what those who succumb die of. Most of the victims developed flu over a week or so, which progressed to severe and ultimately fatal pneumonia.

The virus emerged more than a month ago in Mexico, where the suspected death toll stands at more than 100, but only a fraction of these cases have been confirmed by lab tests. Mexican health officials do not have a clear idea how many people are infected more mildly, making it almost impossible to gauge how lethal the virus is.

There is no clear explanation of why the virus has killed only in Mexico. There is a chance that those who died already had other infections, or simply received worse healthcare. It is also possible that the virus was more virulent when it emerged, and has now mutated into a more transmissible, but less potent, strain.

Many of these uncertainties will only become clearer as the infection spreads throughout America and other countries.

A question yet to be answered is how many people each infected person passes the disease on to, and how this figure changes as time goes by.

Fresh samples of the virus are due at the WHO's flu laboratory at the National Institute for Medical Research in north London on Wednesday. The virus, which will be handled as an exotic pathogen in high-security labs, will be injected into ferrets to investigate how their immune systems respond.

So far, the spread of the disease has mirrored major transport routes between Mexico and its northern neighbour. Mexican commuters travelling into Texas and California may have taken the virus across the border, while Americans and other nationalities on holiday in the country brought it back to their own regions via Mexico City.

As soon as the infection crossed the border, it was picked up and analysed quickly, and that crucial information was shared immediately with the WHO.

Fresh vaccines to combat the infection could take between four and six months, by which time the first wave of the disease could be over. In 1918, when Spanish flu killed tens of millions, there were two subsequent waves, which were far more lethal than the first.

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